function remSize() {
  var deviceWidth = document.documentElement.clientWidth || window.innerWidth;
  if(deviceWidth >= 750) {
    deviceWidth = 750; 
  }
  if(deviceWidth <= 320) {
    deviceWidth = 320;
  }
  document.documentElement.style.fontSize = (deviceWidth/7.5) + 'px';
  document.querySelector("body").style.fontSize = 0.3 + "rem";
}

remSize();

window.onresize = function() {
  remSize();  
}